Add clang.arc.used to ModuleHasARC so ARC always runs if said call is present in a module.

clang.arc.used is an interesting call for ARC since ObjCARCContract
needs to run to remove said intrinsic to avoid a linker error (since the
call does not exist).

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@178369 91177308-0d34-0410-b5e6-96231b3b80d8